Inactivated whole organisms have been used in successful vaccination since the late nineteenth century. In more recent times, vaccines involving the administration of extracts, subunits, toxoids and capsular polysaccharides have been employed. Since genetic engineering techniques have been available, the use of recombinant proteins has been a favoured strategy, obviating many of the risks associated with use of purified proteins from natural sources.
Early vaccine approaches were based on the administration of proteins which stimulated some aspect of the immune response in vivo. Subsequently it was appreciated that immune responses could also be raised by administration of DNA which could be transcribed and translated by the host into an immunogenic protein.
The mammalian immune response has two key components: the humoral response and the cell-mediated response. The humoral response involves the generation of circulating antibodies which will bind to the antigen to which they are specific, thereby neutralising the antigen and favouring its subsequent clearance by a process involving other cells that are either cytotoxic or phagocytic. B-cells are responsible for generating antibodies (plasma B cells), as well as holding immunological humoral memory (memory B-cells), i.e. the ability to recognise an antigen some years after first exposure to it eg through vaccination. The cell mediated response involves the interplay of numerous different types of cells, among which are the T cells. T-cells are divided into a number of different subsets, mainly the CD4+ and CD8+ T cells.
Antigen-presenting cells (APC) such as macrophages and dendritic cells act as sentinels of the immune system, screening the body for foreign antigens. When extracellular foreign antigens are detected by APC, these antigens are phagocytosed (engulfed) inside the APC where they will be processed into smaller peptides. These peptides are subsequently presented on major histocompatibility complex class II (MHC II) molecules at the surface of the APC where they can be recognised by antigen-specific T lymphocytes expressing the CD4 surface molecules (CD4+ T cells). When CD4+ T cells recognise the antigen to which they are specific on MHCII molecules in the presence of additional adequate co-stimulatory signals, they become activated and secrete an array of cytokines that subsequently activate the other arms of the immune system. In general, CD4+ T cells are classified into T helper 1 (Th1) or T helper 2 (Th2) subsets depending on the type of response they generate following antigen recognition. Upon recognition of a peptide-MHC II complex, Th1 CD4+ T cells secrete interleukins and cytokines such as interferon gamma thereby activating macrophages to release toxic chemicals such as nitric oxide and reactive oxygen/nitrogen species. IL-2 and TNF-alpha are also commonly categorized as Th1 cytokines. In contrast, Th2 CD4+ T cells generally secrete interleukins such as IL-4, IL-5 or IL-13.
Other functions of the T helper CD4+ T cells include providing help to activate B cells to produce and release antibodies. They can also participate to the activation of antigen-specific CD8+ T cells, the other major T cell subset beside CD4+ T cells.
CD8+ T cells recognize the peptide to which they are specific when it is presented on the surface of a host cell by major histocompatibility class I (MHC I) molecules in the presence of appropriate costimulatory signals. In order to be presented on MHC I molecules, a foreign antigen need to directly access the inside of the cell (the cytosol or nucleus) such as it is the case when a virus or intracellular bacteria directly penetrate a host cell or after DNA vaccination. Inside the cell, the antigen is processed into small peptides that will be loaded onto MHC I molecules that are redirected to the surface of the cell. Upon activation CD8+ T cells secrete an array of cytokines such as interferon gamma that activates macrophages and other cells. In particular, a subset of these CD8+ T cells secretes lytic and cytotoxic molecules (e.g. granzyme, perforin) upon activation. Such CD8+ T cells are referred to as cytotoxic T cells.
More recently, an alternative pathway of antigen presentation involving the loading of extracellular antigens or fragments thereof onto MHCI complexes has been described and called “cross-presentation”.
The nature of the T-cell response is also influenced by the composition of the adjuvant used in a vaccine. For instance, adjuvants containing MPL & QS21 have been shown to activate Th1 CD4+ T cells to secrete IFN-gamma (Stewart et al. Vaccine. 2006, 24 (42-43):6483-92).
Whereas adjuvants are well known to have value in enhancing immune responses to protein antigens, they have not generally been used in conjunction with DNA or DNA-based vector vaccination. There are several hypotheses as to why adjuvants have not been used in conjunction with DNA-vector based vaccines. Indeed, interferences between the adjuvant and the vector may have an impact on their stability. In addition, one might expect that adding an adjuvant to an attenuated vector could increase the reactogenicity induced by such product. Finally, increasing the immunogenicity of a DNA-vector based vaccine may lead to an enhanced neutralizing immune response against the vector itself, thereby precluding any boosting effect of subsequent injections of the same vector-based vaccine. In fact, in a vaccination protocol directed towards protection against P. falciparum infection, Jones et al (2001, J Infect Diseases 183, 303-312) have reported an adverse outcome after combining DNA, recombinant protein and adjuvant as a boosting composition following a prime by DNA. Indeed, the levels of parasitemia were significantly lower in a group in which the boosting composition contained protein and adjuvant only. It was concluded that use of the combination of DNA, recombinant protein and adjuvant in this protocol adversely affected the outcome on parasitemia as well as antibody responses.
On the other hand, there has been a report of enhancement of the efficacy of an adjuvanted DNA-based vector vaccine (Ganne et al. Vaccine (1994) 12(13) 1190-1196). In particular, the enhanced efficacy of a replication-defective adenovirus-vectored vaccine by the addition of oil adjuvants was correlated with higher antibody levels but the impact on CD4 and CD8 T cell responses was not reported.
The use of an apathogenic virus as an adjuvant has been disclosed in WO2007/016715. It was not mentioned that said virus could contain any heterologous polynucleotide.
It is generally thought that stimulation of both CD4+ and CD8+ cells are needed for optimal protective immunity, especially in certain diseases such as HIV infection/AIDS. In order to induce an optimal immune response either prophylactically or therapeutically, stimulation of both CD4+ and CD8+ cells is desirable. This is one of the main goal of “prime-boost” vaccination strategies in which the alternate administration of protein-based vaccines (inducing mostly CD4+ T cells) with DNA-vector based vaccines, i.e. naked DNA, viral vectors or intracellular bacterial vectors such as listeria, (inducing mostly CD8+ T cells) or vice versa most likely activates both CD4+ and CD8+ T cell responses.
However, although prime-boost vaccine strategies may generally give rise to a greater or more balanced response, the requirement to vaccinate on more than one occasion and certainly on more than two occasions can be burdensome or even unviable, especially in mass immunization programs for the developing world.
Furthermore, as already mentioned above, it is often not possible to boost the viral vector component because of immunity that may have been raised against the vector itself.
Thus the objects of the invention include one or more of the following: (a) to provide a complete vaccination protocol and a vaccine composition which stimulates the production of CD4+ and/or CD8+ cells and/or antibodies and in particular which obviates or mitigates the need for repeated immunizations; (b) to provide a vaccination protocol and a vaccine composition which better stimulates production of CD4+ cells and/or CD8+ cells and/or antibodies relative to vaccine compositions containing an immunogenic polypeptide alone or a polynucleotide alone or relative to a conventional prime-boost protocol involving separate administration of immunogenic polypeptide and polynucleotide; (c) to provide a vaccine composition which stimulates or better stimulates Th1 responses; (d) to provide a vaccine composition and vaccination protocol in which required doses of components, especially viral vectors, are minimised; and (e) more generally to provide a useful vaccine composition and vaccination protocol for treatment or prevention of diseases caused by pathogens. By “better stimulates” is meant that the intensity and/or persistence of the response is enhanced.